Rigify-armature won't attach to mesh
If you select your body mesh and go to the vertex groups, you will see that you have two "series" of vertex groups: one has been created by Rigify parenting (all thoose starting with the suffix "DEF"), the other created by another rig. The DEF vertex groups have no weights assignements (so they don't act), while the others do have weights, but they don't "talk" with rigify bones because they have non corresponding names.

There are three different ways of repair:

1) delete all "DEF" vertex groups, then rename all others vertex groups, i.e. "lindex3" shall be renamed "DEF-f_index.03.L" (I know, it's a pain".

2) rename the deforming bones of Rigify (the opposite, but another pain).

3) Delete all existing vertex groups and redo the weighting. If you want to use Automatic weighting you will have to temporarily detach the head from the body, because in the head region you will get the "Bone heat failing" error, and probably this is the reason why you did the parenting but only empty vertex groups were created.

BTW, it's correct to parent the mesh to "Rig", not "Metarig".